I've learned a few signs of Japanese Sign Language from watching tv dramas, and some from http://www.necsoft.com/syuwa/back.html. I think to learn much else I'd have to find someone here who knows it (I've heard of an ASL instructor in the Bay Area who does), or go to Japan for a while. The latter would be better, as there are big regional differences in dialect, and I could travel around learning. Sounds fun!

Their fingerspelling follows their whole syllabary, which is also covered by hiragana and katakana.

As described in the wiki, voiced consonants which would normally get the ten ten marks (they look like a double quote ") in either hiragana or katakana, get moved towards the right. For example, the "ka" handshape moved towards the right becomes "ga." "Pa," "pi," "pu," "pe," and "po" (given a circle in hiragana and katakana) are made by moving "ha," "hi," "fu," "he," and "ho" up, while small vowels and small "tsu" are made by moving the handshape towards the signer.

So, I learned the 46 handshapes, and the rules for modifying them, to be able to fingerspell anything in Japanese. Which I've been doing, every time I run into a Japanese word when I'm out and about (I don't count every word I hear watching anime, heh). A girl named Nozomi prepared my sandwich at a cafe yesterday...